Just look for cracked pieces of wall while inside any puzzle room during the campaign, there are a lot of rooms but if you know what to look for, they are easy to spot.
Sadly, I have to recognize, the secrets in the first chapter can be more difficult to spot as the tiles blend quite well.
I'm working on my new project and I'm hoping to rebuild the story of this game there so, I can't dedicate time to update this project anymore but they should be easy to track and find if you make a map of the routes you followed.
Sorry I can't help you more and that the low resolution of the image, should be easier with the crisp pixel art within the game. Good luck.
